Preliminary observations have suggested that the cyclic nucleotides might be important in the regulation of normal and malignant tissue growth, but measurements of the levels of cyclic AMP and cyclic GMP in growing cells have produced inconsistent results. Recent immunohistochemical studies of the cyclic nucleotides indicate that both cyclic AMP and cyclic GMP exist in discrete loci within cells, raising the possibility that important changes in cyclic nucleotide concentration might occur in critical cell loci, which might not be reflected in measurements of the total tissue content of these nucleotides. Interestingly, cyclic GMP is found in association with nuclear chromatin, while cyclic AMP is not. In preliminary studies in freshwater sponges, using an improved immunofluorescent technique, we have found an increase in cyclic GMP bound to nuclear structures at times of rapid cellular growth and proliferation, suggesting that this nucleotide may be involved in growth regulation. We plan to continue our studies of the freshwater sponge during cellular growth and differentiation, and to apply our immunohistochemical and radioimmunoassay techniques to a study of the cyclic nucleotides in regenerating liver following partial hepatectomy and regenerating kidney following unilateral nephrectomy in the rat. Further, the dimethylbenzanthracene-induced mammary carconoma in the rat, the growth of which is sensitive to hormonal influence, will be similarly studied during induction, growth and regression.